1. Field of the Invention
The present invention concerns a method of producing reduced iron agglomerates by reducing iron oxide agglomerates incorporated with a carbonaceous material.
2. Description of the Related Art
The MIDREX method is well-known as a method of producing reduced iron. In the MIDREX method, a reducing gas produced from natural gas is fed through a tuyere into a shaft furnace and allowed to rise therein for reduction of iron ores or iron oxide pellets charged therein, to thereby produce reduced iron. However, since the method required a supply, as a fuel, of a large amount of high-cost natural gas, the location of a plant utilizing the MIDREX method is limited to a region producing natural gas.
In view of the above, a certain type of method for producing reduced iron has become of interest, in which relatively inexpensive coal can be used instead of natural gas as a reducing agent. A method of producing reduced iron is disclosed, for example, in U.S. Pat. Nos. 3,443,931 and 5,601,631 being hereby fully incorporated by reference, in which powdery ores and carbonaceous materials are mixed and pelletized and then reduced by heating in a high-temperature atmosphere to thereby produce reduced iron. This method has advantages in that coal can be used as a reducing agent, as well as powder of ores can be used directly, reduction can be performed at a high rate and the carbon content of products can be regulated.
In the method described in the U.S. patent applications, dried iron oxide agglomerates are charged in a traveling hearth heating furnace such as a rotary hearth furnace, heated while moving in the furnace and the iron oxide agglomerates are reduced by a carbonaceous material.
Reduction of the iron oxide agglomerates by the carbonaceous material proceeds from the surface of the agglomerates in view of the heat transfer. Accordingly, in the latter half of the reduction process, while metallic iron is deposited on the upper surface layer of the agglomerates, the reducing reaction has not yet proceeded sufficiently in the central portion or the lower surface where the temperature is low, and the quality of the reduced iron is not satisfactory. In addition, while the temperature for the rotary hearth furnace is controlled by a combustion burner or secondary combustion of a combustible gas released from the agglomerates, it is necessary to burn the combustible gas released from the fuel and the agglomerates substantially completely at the exit of the furnace in order to decrease the fuel unit. However, unless an appropriate reducing atmosphere is maintained, the agglomerates are exposed to an oxidizing gas, particularly, in the latter half of the reducing zone and the metallic iron at the surface layer of the agglomerates is reoxidized to deteriorate the quality of products.
Further, the iron oxide agglomerates should be of a size suitable to operation conditions. Those of a size suitable to the operation conditions attain a high quality after reduction, whereas those of a size smaller than the appropriate size are reoxidized due to excess heating, while those of a size greater than the appropriate size are reduced only insufficiently due to insufficient heating. In addition, if the particle size of the agglomerates is scattered, both of the reduction ratio and the strength are deteriorated as a whole product.